Coupling circuit



Aug. 13, 1940- T. s. SKILLMAN 2,210,956

COUPLING CIRCUIT Filed Nov. 22, 1957 Patented Aug. 13, 1940 UNITED STATES PATENT vOFFICE 2,210,956 COUPLTNG CIRCUIT Application November 22, 1937, Serial No. 175,983 In Germany November 23, 1936 3 Claims.

My invention relates to a coupling circuit for connecting a generator to a load.

When a plurality of electric circuits are fed from a common generator, the internal resistance f the latter brings about an undesirable mutual coupling of the circuits. This take place for instance, in carrier wave telephony-or-telegraphy, in which a plurality of carrier wave systems, each consisting of a certain number of channels, are operated simultaneously. It is desirable to feed the channels of the various systems operating at the same carrier wave frequency from a common carrier generator, but this introduces diiculties. More particularly, the internal resistance of the common generator involves crosstalk from the channel of sponding channel of the just the degenerative back-coupling that the amplier has a low internal resistance.

In order that the invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing in which the single figure is a schematic diagram of a circuit embodying the invention.

As shown in the drawing, a generator G having an internal impedance Zi is connected to the which form part of two diiierent carrier wave systems and to which the carrier wave produced by the generator Gis to be supplied.

Without interposition of ampliiier A between the generator and modulators M1 and M2, a coupling would be set up across the internal impedance Zi The same would apply if an amplifier without degenerative backcoupling were used, because the internal resistance of the amplifying tube would act as a coupling impedance.

According to the invention ampliier A is backcoupled in a degenerative manner. As shown the amplifier comprises a thermionic tube 5 having a grid 9, a plate lll, and an incandescible cathode ll, supplied with heating current by a battery I6. Plate I il is connected through a condenser l to one end of the primary winding l2 of a coupling transformer 3; the other end of this winding being connected to the cathode Connected between plate l and cathode ll is a resistance 6 and a source of plate voltage l5. Winding I2 is a potentiometer whereby the coupling between the two carrier wave systems will be materially reduced.

I have found that degenerative back-coupling involves variation of the internal resistance of the amplifying tube. For example, if tube Without back-coupling has an internal resistance R11 and an amplifying factor g, the internal resistance of the amplifying tube with degenerative back-coupling may be represented by the Iormula:

the electric circuits connected to the output circuit of the amplier. For instance, if R1=5,000 ohms, 9:25, and ,6:20, then Rif will be equal to i0 ohms.

The coupling between the load circuits is further decreased by inserting resistances R. in the output circuit as permissible in view of What I claim is: 1. A

the prior art.

reducing the coupling between said electric circuits comprising a degenerative back-coupled amplifier connected be- ,55

and comprising an amplifying tube having an 10 anode, a' cathode and a grid, a plate circuit for said tube insulating a coupling transformer having a primary winding and a secondary Winding connected to said loads, a potentiometer connected across said primary Winding, and a grid circuit for said tube including said generator and a portion of said potentiometer, said portions being so selected that the amplifying tube vhas a low internal resistance.

THOMAS SAMUEL SKILLMAN.

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